The present invention relates to modification of borehole sidewalls, in particular in connection with drilling of long boreholes. The invention is particularly adapted for extended reach drilling, ERD of wells such as offshore oil wells. Especially, the invention concerns an apparatus and method making a spiral shaped rifle inside a wellbore. The modified borehole is not a circular borehole with a uniform diameter along the length of the hole, but rather a hole with a non circular cross section, or a hole with varying diameter along the length. The apparatus of the invention may be used for milling the sidewalls of an all ready drilled hole, or may form a part of a unit providing simultaneous milling and drilling.
A solution for creating of non-circular boreholes in rocks for use with geothermal heat pump applications, and for increasing wellbore support in applications such as horizontal oil and gas drilling, is shown in US patent application 2008/0093125. The system uses a non contacting drilling system which in one embodiment uses a supersonic flame jet drilling system with a movable nozzle that swings between pivot points.
U.S. Pat. No. 4,629,010 shows a process and device for sinking wells. The device includes a pair of rotary digging heads with horizontal axes mounted on supports for movement of such axes towards and away from each other between a central position close to the axis of a well and a peripheral position adjoining the wall of the well being formed.
In for instance oil well drilling is well construction is an important cost driver in oil and gas field development, and extended reach drilling, ERD, is a key challenge. These types of drilling include extended reach far out from infrastructure and multiple targets, and may involve slim hole infill drilling, through tubing rotary drilling (TTRD), coil tubing drilling (CTD) and casing while drilling (CWD).
Extended reach drilling (ERD) is limited due to drill string friction causing excess drag and torque. This again may lead to drill string buckling and twist off.
Furthermore, increased pressure loss along the drill string may cause excessive pressure to the formation equivalent circulating density (ECD). This may again lead to borehole instability and lost circulation, poor well cleaning and stuck drill string. Drill string/borehole wall contact may also cause mud cake damage. A rotating drill string whirling in a borehole may cause damage to a mud cake continuously, further resulting in mud filtration loss and invasion zone around the borehole. With the present invention, this contact is reduced, and this causes less mud cake damage, less filtration losses and less wellbore instability. The contact can be reduced as much as 75% while a buckling limit of a drill string is maintained. Increased mud invasion may lead to borehole instability, well collapse and formation damage in a hydrocarbon reservoir. The present invention may maintain the same borehole clearance and thereby the same buckling capacity of a drill string in the borehole. Less invasion zone in a reservoir leads to reduced skin effect, improved production rate and higher recovery factor.
Fluid loss of the drilling mud is one of the main factors that results in borehole instability in formations. Maintaining the mud cake will decreases the rate of fluid loss. Preventing mud cake damage has a big impact in reducing the volume of fluid loss. Therefore, the present invention reducing the wall contact between a bore hole wall and eg. a drill string, will have a big impact to reduce the borehole instability.
Reduced borehole instability leads to less drilling time and increased operational safety, which are the main challenges issues in the drilling industry.
With a borehole with modified sidewalls according to the invention, it is proposed to increase the reach of wells by reducing the borehole wall friction by reducing wall contact, reducing the pressure loss along the drill string (ECD) by increasing an annulus area, increasing borehole stability by reducing wall contact of eg. a drill string, reducing differential sticking of drill string in open hole section, improving hole cleaning due to rotational flow regime in annulus, and reducing surface torque by eliminating drill bit reactive torque. Reducing the circulation pressure loss in an annulus reduces the pressure on the formation and thereby the risk of formation fracture. The total pressure inserted to the formation is equivalent to the hydrostatic pressure of circulating mud with the equivalent density in the annulus, ECD, is one of the main limiting factors in extended long boreholes. With the present invention, the ECD can be reduced, and thereby, the borehole length can be increased.
Reducing drill string/wall contact in borehole may again lead to less mud filtration loss and less borehole instability. Time is a key factor on volume of filtration loss.
Friction factor is an empirical parameter, affected by different variables: Sliding velocity, temperature, vibration, surface quality, contact area and extent of contamination. The friction laws for lubricated surfaces are considerably different than for dry surfaces. In lubricated surfaces the frictional resistance is almost independent of the specific load.
Increasing flowing area leads to less pressure loss (ECD) in the annulus.
            Δ      ⁢                          ⁢              p        annulus              =                            48          ⁢                                          ⁢                                    μ              pl                        ·            L            ·                          v              _                                                            (                                          d                h                            -                              d                Dp                                      )                    2                    +                        6          ⁢                                          ⁢          L          ⁢                                          ⁢                      τ            o                                                d            h                    -                      d            Dp                                    ECD    =                  ρ        m            +                                    Δ            ⁢                                                  ⁢                          P              annu                                +                      Δ            ⁢                                                  ⁢                          P              acceleration                                +                      Δ            ⁢                                                  ⁢                          P              cuttings                                      gh            
Initial calculation models suggest 300% extended reach and 88% reduction in pressure loss per unit length. Additional benefits may include less drilling problems (less NPDT), reaching new targets and environmental benefits. This also results in saving costs.
Creating spiral grooves around a main borehole improves the flow regime with respect to drill cuttings transport in an annulus.
Lateral fluid movement in the spiral grooves and rotational flow regime will improve drill cuttings transportation compared to a circular annulus. In addition, more clearance below a drill string increases fluid velocity on a lower side of the borehole. Higher fluid velocity in this section is a key issue to increase cuttings transport reduce particle deposition.
Typical flow simulation with equivalent conditions show that the returning mudflow regime in a rifeled well annulus will be different compared to conventional annuluses and hole cleaning in ERD wells will be improved.
Higher fluid velocity below the drillpipe in the horizontal section leads to improved well cleaning and less settled cuttings. Better borehole cleaning will reduce the settled cuttings and excess drag and torque. Using larger diameter drillpipe for increased mechanical properties is also an option in some cases to extend the drilling reach.
A study of pressure loss in the different cases and comparing the conventional and a modified drilling system shows that the borehole cleaning is much better in modified wells.